The present invention relates to electronics and electronics circuits. More specifically, the present invention relates to amplifier electronic circuits and comparator electronic circuits.
A comparator is generally considered to be a differential input electronic device having an output which toggles between a high and low output signal level depending upon a comparison between two input differential signals. The output signal will obtain one of two discrete states depending on which input signal has a higher voltage signal in comparison to the other input signal.
The comparator is supplied power by a positive power reference e.g. five volts DC, and a negative power voltage reference e.g. zero volts DC. The output signal will toggle between a voltage signal that approaches the positive power reference and another voltage signal that approaches the negative power reference.
The comparator includes an amplifier stage amplifying the voltage difference between the input differential signals. The output signal toggles between high and low output signals because of the high gain of the amplifier stage. The comparator can be considered to be an amplifier operating in an open loop configuration, that is without negative feedback controlling the linear operation of the amplifier. Hence, the comparator digitally operates without the need for compensation capacitors typically, used to control linear amplification through the unity gain bandwidth of the amplifier.
The difference between the input signals is amplified by the amplifying stage internal to the comparator which then provides an amplified output signal. The amplifier stage has a large gain so that a relatively small differential input signal, e.g. one microvolt, will cause the output of the comparator to approach one of the power references.
Typical differential amplifier designs include a differential input stage, which is a gain stage, connected to an output gain stage. The gain of the differential amplifier is the product of the gain of the differential input stage and the gain of the output gain stage.
One problem associated with conventional two stage voltage gain amplifiers are parasitic effects in which large varying voltage signals produce current flow in parasitic capacitance thereby reducing the bandwidth of the amplifier and thereby increasing power consumption. Hence, amplifiers which operate on internal voltage signals with large amplitude variation tend to have lower slew rates and bandwidths. Hence, conventional two stage amplifier internal to comparators have a substantially limited operating frequency with increased power dissipation.
Hysteresis, which acts to retard rapid toggling of the output signals has been a feature long embedded into comparators. Typically, the hysteresis is measured by that minimum voltage change which must be exceeded to the cause toggling of the output signal immediately subsequent to a prior toggling. The hysteresis function is better understood by way of two examples. In a first example, once the input voltage of one different input increases above the other causing an output toggling, that input voltage must decrease below the other by at least a hysteresis value, e.g. ten millivolts, before the output will toggle back to the original output level.
In the second example, once the input voltage of one differential input decreases below the other causing an output toggling, that input voltage must increase by at least a hysteresis value, e.g. ten millivolts, before the output will toggle back to the original output level.
Hysteresis in comparator has been used to prevent erratic oscillations of the output signal when the two input signal are approximate to each other. For example, in a comparator without hysteresis, when two input signal are approximate to each other, the output will obtain a given level, but, the output may oscillate in an erratic manner if unwanted superimposed noise signal is present on one of the input signal. In this example, if the comparator has a hysteresis value, then the unwanted superimposed noise signal would have to be larger than the hysteresis value in order to cause the aforementioned erratic oscillations. Hence, in this example, hysteresis in a comparator is used to prevent erratic unwanted oscillations due to unwanted superimposed noise.
Those skilled in the art well appreciate the various beneficial effects of hysteresis in comparators. An example of a commercial comparator is that provided by MOTOROLA Inc. part number MC14574 using P channel and N channel enhancement MOS devices in a single monolithic integrated electronic circuit. One problem associated with conventional comparators is the precision at which the hysteresis value is maintained through varying temperature and power supply references.
Designers of comparator and amplifier circuits have strived to continually increasing the speed of the comparator circuits while controlling the hysteresis and while limiting the power consumption of the circuits. The speed, hysteresis and power consumption features of the amplifier and comparator circuits are improved upon using the herein disclosed invention.